\chapter{Introduction}

Optical mesh networks have underwent a rigorouls evolution cycle. With the introduction of optical (D)WDM modulation, 
they have begun to replace older SONET/SDH ring-based models due to being able to maintaining 
fast recovery times in case of failure while also providing a more robust capacity efficiency 
model and cheaper deployment and upgrade scenarios (TODO ref ken liu IP/WDM). The latest emerging technologies in such 
networks aim to eliminate the costly process of opto-electronic conversions by 
introducing Reconfigurable Optical Add-Drop Multiplexer (ROADM) systems.
%Optical networks that incorporate ROADM systems are said to be transparent because of their capability to switch signals on the wavelength level without the need for OEO conversions. 
In addition, ROADMs provide the capability of dynamic reconfiguration of end-to-end lightpaths making optical mesh networks a highly scalable and dynamic environment.

\section{DAS-4 Photonic Cross-Connect network}

The Distributed ASCI Supercomputer 4 (DAS-4) is a computational grid comprised of wide-area, geographically-disjoint clusters used for computer and science research in the Netherlands. DAS-4 consists of eight clusters (TODO: correct num of clusters?) located in the campus facilities of four Universities - UvA and VU in Amsterdam, Leiden University and TU Delft - with one located in the Netherlands Institute for Radio Astronomy (ASTRON). The primary usages of the DAS-4 testbed are Grid-based applications and distributed systems whose data sets target research data collected from disciplines such as Computer Science, Radio Astronomy and genome sequencing in Biological studies. 

DAS-4's clusters are connected together via a transparent all-optical network which uses SURFnet's 
(a Dutch National Research and Education Network) DWDM infrastructure. A Degree-4 Fast Photonic Cross-Connect (FPX) (TODO ref) provides Layer 0 wavelength (Fig x a)services for dynamic link establishment and capacity provisioning. Therefore, routed and routing network traffic is transported  between Layer 3 nodes at each cluster without leaving the optical layer. By incorporating particular wavelength selective switches (WSS) (Fig x b), the FPX supports dynamic reconfiguration in addition to remote management. Those capabilities enable DAS-4's network to cope with multi-cluster workloads on-demand and fulfill the requirements of communication-intensive applications.

\begin{figure}[h]
	%\begin{center}
			\includegraphics[width=4.7in]{/home/madave/Dropbox/school/RP2/das4-routing/report/figures/fpx.jpg} 
	%\end{center}
	\caption{Degree-4 Photonic Cross-Connect. Image courtesy of TU Eindhoven and SURFnet}
\end{figure}
\vskip2em

Dynamic lightpath reservation, provisioning and release in the network are done by OpenNSI (TODO ref), a generic implementation of the Open Grid Forum (OGF) Network Service Interface protocol(NSI) (TODO ref). By coupling OpenNSA with the remote management facilities offered by the Finisar DWP50 WSS (TODO ref), 
the DAS-4 network allows granular control over lightpath reconfiguration which can be carried out in a sealmess fashion by both researches and autonomous application code.

\section{Research Question \& Approach}

Transparent optical mesh networks facilitate dynamic end-to-end lightpath provisioning. Coupled with DWDM transport, such networks address constraints such as path and capacity allocation in an automated way. Achieved at Layer 0, dynamic lambda provisioning introduces additional considerations in terms of the existing IP overlay network that flows above it, namely dynamic topology reconfiguration. The bottom part of Fig x a. illustrates the two main considerations that arise. 
Lightpaths (links) connecting Layer 3 nodes may be allocated at will, which results in nodes changing their position in the topology and therefore their neighbors. Given nodes VU, ASTRON and UvA, where the dots of nodes are synonymous to the routers at each respective cluster, it can be seen that in one topology configuration nodes VU and UvA share a direct lightpath  whereas in another configuration they may be separated by the intermediate hop ASTRON. In addition, given nodes VU and ASTRON, a single lightpath may be assigned between the nodes with the purpose of providing connectivity at default line speeds, whereas in an alternate configuration multiple lightpaths may be assigned between them in order to achieve higher link speeds. In such a configuration, a particular wavelength is assigned to each pair of interfaces and multiple optical interfaces at each node need to be bundled which results in logical addressing, routing and link aggregation changes.

Each cluster of the DAS-4 network contains a router, or otherwise stated - a head node. Head nodes, represented by multi-purpose Linux platforms, are equipped with separate interfaces for connecting to the SURFnet StarPlane network (TODO ref), the Internet and the computational equipment part its respective cluster. Four 10Gbps optical transceivers will be used for connecting to StartPlane at each cluster. Currently, the IP routing problem introduced by the dynamic lightpath allocation process is approached by using configuration scripts run manually on head nodes after each dynamic reconfiguration of the underlying optical network. The scripts handle interface IP addressing, insertion of rules for static routing and capacity provisioning via Link Aggregation Control Protocol (LACP).

In order to enable routed protocols to cope with the dynamic changes occurring in the optical layer without the need for intervention from Network Administrators, a system is needed capable of sensing changes in the optical layer, adjusting routes to network prefixes and providing link capacity by relying on data obtained from protocols at the Data link and Network layers. Therefore the following research question is defined: 

\begin{center}
\emph{What are the most suitable technologies for addressing routing and capacity provisioning in DAS-4's dynamically reconfigurable network?}
\end{center}

Subsequently, focus falls upon the following subquestions:

\begin{itemize}
    \item \emph{What is the applicability of Transport Layer protocols such as SCTP and MPTCP?}
    \item \emph{What is the applicability of routing protocols such as OSPF?}
    \item \emph{What is the applicability of link-bonding protocols such as LACP?}
\end{itemize}

To be able to fulfill the task, the research was performed in three phases. First, a theoretical study was conducted on the different protocols and their ability to provide prefix distribution and capacity provisioning. Knowledge gained during the phase was instrumental in both defining the research questions in addition to conducting the next phase - a test study on the protocols operational characteristics. In this phase, a test setup was created with the purpose of comparing the different methods of capacity provisioning, which included two test machines with multiple network interfaces and a managed switch. A series of tests were conducted by using LACP - the currently used technology in DAS-4 for increasing network capacity - as a baseline. The tests aim to examine whether OSPF load-balancing and MPTCP/SCTP multi-homing perform as well as LACP.

\subsection{Related work}
The academic community provides extensive research on the topic of IP over reconfigurable WDM [3] [4].
However, no research has been conducted in the direction of evaluating the applicability of different
technologies that would enable IP to retain connectivity in such networks where dynamic reconfiguration
exists. In addition, IP over WDM networks may differ in the way the optical layer and the above IP layer
interact. In the case of DAS-4, the IP layer is unaware of the semantics of the optical layer with clear
demarcation points in between the two. The scope of the project will examine applicable technologies that
would enable IP to retain functionality with regards to a network setup in which IP uses an overlay control
model.


\subsection{Thesis outline}

The rest of the paper is organized as follows. Chapter 2 presents the DAS-4 network. Chapter 3 contains a detailed overview of protocols by focusing on the characteristics that make them suitable for the project in addition to side-effects brought by them. Chapter 4 includes testing performed with the different protocols. Chapter 5 elaborates on the overall architecture of the automation process and difficulties that arise with it.

